The present invention relates to the field of display devices and in particular to screens that can enhance the viewing angle, especially for liquid crystal displays (LCD).
Display devices as for example projection display devices, off screen display devices and direct view displays are known. See for example, Zimmerman et al U.S. Pat. No. 5,481,385. Such displays are used in a wide range of applications which include computer terminals, airplane cockpit displays, automotive instrument panels, televisions and other devices that provide text, graphics or video information. Conventional direct view displays as for example liquid crystal displays suffer from a number of inherent disadvantages. For example, at high viewing angles (large angles from the direction normal to the surface of the display), such displays suffer from low contrast and changes in visual chromaticity as the viewing angle changes.
For most display devices it is important that the operator can view the display at various off-axis angles. For example, in an aircraft with right and left crew seats (pilot/copilot), the pilot may need to look across the cockpit to read the copilot""s displays. In this situation, it is desirable to have a large ( greater than 60 degree) off-axis viewing angle. Certain types of displays (e.g. LCDs) have a limited off-axis ( less than 20 degrees) unless external compensation is provided.
In Zimmerman, U.S. Pat. No. 5,481,385, issued Jan. 2, 1996, a viewing screen is placed between a liquid crystal display (LCD) and a person viewing the display. It is desirable in the art to pass collimated light through a LCD display in order to produce an image. Such collimated light has a fairly low scatter angle (approximately 5 to 10 degrees) which results in undesirable off-axis viewing for the display. The viewing screen of Zimmerman, which includes a plurality cone-shaped optical waveguides on the viewer""s side of the screen, is shown in FIG. 1. The viewing screen comprises a plurality of tapered waveguides 11 that are disposed on a transparent planar substrate 12, such as glass. Each tapered waveguide has a light input surface 13 (not seen in FIG. 1) through the substrate 12, a light output surface 14 and tapered sidewalls 15. These optical waveguides are constructed from a photopolymeric material such as acrylic.
FIG. 2 illustrates the internal light reflections (i.e. light path) through such a tapered waveguide. Light rays 101 enter a tapered waveguide 11 at the light input surface 13 and propagate through the tapered waveguide 11 via a number of reflections off the tapered sidewalls. At the first reflection, the scatter angle of the light ray 101 is increased from A to A+B, where B is the taper angle of the sidewall. At the second reflection 22 the scatter angle is increase to A+2B. In general the scatter angle of the light output from a tapered waveguide can be expressed as A+(n*B), where xe2x80x98nxe2x80x99 is the number of reflections from the tapered sidewalls before the light ray 101 exits the tapered waveguide 12. Note that the angle of incidence is equal to the angle of reflection for each light ray 101.
A transparent waveguide, such as shown by Zimmerman, propagates light though total internal reflection (TIR) in the same manner as a fiber optic cable. TIR requires that the incident angle of the light is less than a critical value determined by materiel properties of the optical medium (e.g. acrylic). FIG. 3 illustrates a plurality of light rays 101 propagating through a tapered waveguide, as taught by Zimmerman, to a output light spread angle 31. After a limiting number of reflections, the scatter angle of the light ray 101 will exceed a critical incident angle for the waveguide optical medium and the light rays will exit at side points 32 on the tapered waveguide 11 instead of reflecting from the tapered surface 15. Zimmerman teaches that it is desirable to absorb light that escapes the waveguide 11 by filling the interstitial region between the waveguides with a light absorbing material, such as lampblack.
Typically a tapered cone waveguide is constructed of an optical medium such as glass or a photopolymeric material such as acrylic. Zimmerman teaches a photopolymerization process, using ultraviolet (UV) light from a mercury (Hg) or xenon (Xe) source that is especially suitable for manufacture of a tapered cone waveguide. The exposure sandwich used mask/methanol/PET/photopolymer/clear glass top plate. The exposure sandwich was developed and then hard UV baked. A mask with 50 micron holes and 5 micron lines produces tapered cones that are 200 microns high and have tips (light exit area) that are 20 micron wide. The cones are typically fused to each other at a depth of 160 microns and have a 12-degree sidewall taper.
Light rays 101 enter the tapered cone waveguide 11 at the light input surface 13 and exit through the light output surface 14. As the light ray 101 propagates through the optical medium 12, it is reflected from the sidewalls 15. Geometry for light rays 101 at the left and right extremes of the input spread angle are illustrated. After a certain number of reflections, the light ray incidence angle exceeds the refraction index for the optical material and escapes from the waveguide as shown at 32. This results in a limited output light spread angle 31.
Certain improvements to the Zimmerman concept are known including filling the interstitial regions between the waveguides 11 with an optical medium that has a lower refractive index than the refractive index of the waveguides. However this approach will cause undesirable reflections of external light as seen by the viewer. Desired features for avionics displays include high ambient light reflection, high transmission, and asymmetrical output luminance distribution about the display normal.
Our invention provides an array of tapered cones on the viewing side of a display to increase the off-axis viewing angle. Our invention teaches metallic reflective coating is placed on the exterior of the tapered optical waveguides. In addition, the surface of the resultant metallic cone facing away from the waveguide can be processed to form a black light absorbing area facing the viewer. This black area is useful in reducing reflections as seen by the viewer.
Typical use of the invention would be a viewing angle enhancement screen for use with a conventional liquid crystal display (LCD). These LCD displays typically output collimated light (spreads approximately 5 degrees).
An asymmetrical tapered optical waveguide helps promote wider horizontal versus vertical viewing distribution. A tilted cone provides asymmetrical viewing about one axis (vertical). The metallic coated cone provides higher viewing angle.
The hollow metallic cone is a variation of the previous concept with the photopolymer removed. Preferably an overcoat would be placed on the interior metallic surface to prevent corrosion/oxidation.
The direct view display device of this invention exhibits several advantages over known devices. For example, the device of this invention has an asymmetric viewing angle that can be adjusted to match the characteristics of a liquid crystal display to the cross-cockpit viewing requirements of an aircraft.